Viscous fluid(s) (e.g., polymers, plasticizers, colorants, powders, foods, etc.) are often blended with other viscous fluids and/or additives to obtain composite materials having certain desired properties. However, because of their extremely high molecular weight, polymers, for example, are intrinsically difficult to process. In fact, polymer blending has traditionally been accomplished by forcibly melting and mixing the materials together in a batch mixer or extruder, such as single-screw or twin-screw extruders.
Unfortunately, when blending polymers or other viscous fluids in a conventional manner, the morphologies (i.e., the shapes adopted by minor and major components) of the resulting composite cannot be adequately controlled. For example, when blending polymers to form a multilayered film, such as through coextrusion, it is virtually impossible to obtain a film structure that has a large number of thin layers. Some methods have been developed, such as layer stacking, to obtain multilayered films with a relatively large number of layers. However, such methods are inflexible, difficult to control, extremely complicated, and costly to utilize.
In addition to the difficulties currently encountered in forming multilayered films, similar difficulties have also arisen in forming other types of structures from a blend of viscous fluids. For example, when forming fibers from polymers within an extruder, the polymers are simultaneously sheared and melted such that the morphology of the blend often typically forms a dispersion of droplets. In order to form a fiber structure in one component, for example, the sizes of these droplets must be sufficiently large so that viscous forces acting on them can overcome interfacial tension (i.e., for capillary numbers exceeding the critical capillary number). To form such large droplets, the minor phase concentration must be high enough to promote coalescence of small droplets within the extruder.
Thus, at lower concentrations, minor component droplets do not undergo sufficient coalescence before arriving at the die entrance and thus, the small droplets cannot effectively form fibrils. Instead, in such situations, a dispersion of fine droplets is eventually obtained. On the other hand, when the concentration of the minor phase component is larger, coarser droplets and fibrils may eventually form.
In response to some of these difficulties, extruder designers have attempted to provide some control over blend morphology. For instance, extruder designers have provided limited processing flexibility by offering different screw designs, a range of shear rates, and adjustable operating temperatures. However, such design alterations and modifications are time-consuming, costly, and offer relatively no ability to selectively control blend morphology.
In addition, chaotic mixing has also been utilized to improve the blending of polymers. For example, one method for blending polymers using chaotic mixing was described, for example, in two articles entitled xe2x80x9cEmergence of Fibrillar Composites Due to Chaotic Mixing of Molten Polymersxe2x80x9d by Y. H. Liu and D. A. Zumbrunnen (Polymer Composites, Vol. 17, No. 2, April 1996) and xe2x80x9cAuto-Processing of Very Fine-Scale Composite Materials by Chaotic Mixing of Meltsxe2x80x9d by D. A. Zumbrunnen, K. C. Miles, and Y. H. Liu (Composites, Part A, Vol. 27A, No. 1, 1996).
Moreover, another method, as described in xe2x80x9cChaotic Mixing in Extrusion-Based Melt Spinning of Fibersxe2x80x9d by M. Ellison, D. Zumbrunnen, B. Gomillion, and Jiong Wang (National Textile Center Annual Report, http.//www.ntcresearch.org, November 1998) was also developed to form fibers utilizing chaotic mixing. In particular, as shown in FIG. 1, a continuous flow chaotic mixer 110 includes a fixed outer cylinder 112 and two rotating inner cylinders 114 and 116. Two polymers can be provided to the mixer by two conventional extruders 118 and 120. Within the mixer 110, the polymers can be blended by rotation of the cylinders 114 and 116.
Nevertheless, none of the above methods have been totally successful in fully controlling polymer blending to selectively form certain coherent structures (e.g., multi-layered films, fibers, interpenetrating blends, droplet dispersions, and the like) with desired characteristics, such as thin-layered, small diameter, etc.
As such, a need currently exists for an improved method of blending viscous fluids (e.g., polymers) and a method of controlling such blending to obtain certain coherent structures with desired characteristics.